1. Field of the Invention
The present invention relates to a thermoadhesive conjugate fiber which is high in adhesive tenacity after thermal adhesion and extremely small in heat shrinkage after thermal adhesion and to a manufacturing method of the same. In more detail, the invention relates to a thermoadhesive conjugate fiber which despite of low orientation and high elongation, has a satisfactory crimp performance and is provided with satisfactory card-passing properties, high adhesion and low heat shrinkability and to a manufacturing method of the same.
2. Description of the Related Art
In general, a thermoadhesive conjugate fiber represented by core/sheath type thermoadhesive conjugate fibers made of a thermoadhesive resin component as a sheath and a fiber forming resin component as a core is used by a forming a fiber web by a card method, an airlaid method, a wet paper making method, or the like and then melting the thermoadhesive resin component to form fiber-to-fiber bonding. Namely, since an adhesive using an organic solvent as a solvent is not used, discharge of noxious substances is less. Also, since an improvement in production rate and a merit in cost reduction following this are large, thermoadhesive conjugate fibers have been widely used for fiber structures such as fiber cushion and bed mat and nonwoven fabric applications. Furthermore, for the purpose of aiming to improve nonwoven fabric tenacity and to improve production rate of nonwoven fabrics, it is investigated to improve low-temperature adhesion or adhesive strength of thermoadhesive conjugate fibers.
Patent Document 1 discloses a thermoadhesive conjugate fiber obtained by using a terpolymer composed of propylene, ethylene and butene-1 as a sheath component and crystalline polypropylene as a core component and conjugate spinning the both in a ratio of a sheath component weight to a core component weight of from 20/80 to 60/40, followed by drawing in a low draw ratio of less than 3.0 times. It is disclosed that the subject thermoadhesive conjugate fiber has high adhesive tenacity as compared with ones of the related art. However, since such a fiber is low in draw ratio, a uniform tension is not applied between single yarns, scattering in neck deformation is large, and fineness unevenness is generated. Furthermore, there was involved a drawback that heat shrinkage percentage and unevenness of heat shrinkage are large.
Patent Document 2 discloses a thermoadhesive conjugate fiber formed of a thermoadhesive resin component having an orientation index of not more than 25% and a fiber forming resin component having an orientation index of 40% or more by a high-speed spinning method. It is disclosed that the subject thermoadhesive conjugate fiber is strong in adhesion point strength, is molten at lower temperatures and is low in heat shrinkage percentage.
However, in such a fiber, orientation is relatively low; elongation is high; orientation by drawing is insufficient; and orientation crystallization proceeds in high-speed spinning. Accordingly, in a mechanical crimp-imparting method by a crimper with a stuffing box or the like, crimp which has been once imparted is recovered, and fiber-to-fiber entanglement is easy to become worse. Accordingly, the subject thermoadhesive conjugate fiber is poor in card-passing properties. That is, since the web is cut, it is impossible to increase a card-passing speed. Therefore, there was involved a problem that the volume of manufacture cannot be increased in manufacturing nonwoven fabrics. On the other hand, at the time of fiber manufacture, there is a method of strengthening crimp of fibers by performing heating prior to passing through a crimper. However, since the stiffness of fiber is low, the crimp is very fine. Accordingly, since fiber-to-fiber entanglement is excessively strong, the card-passing properties become rather deteriorated. As described above, in thermoadhesive conjugate fibers with low orientation and high elongation, there have not been proposed fibers with satisfactory card-passing properties so far.
Patent Document 1: JP-A-6-108310
Patent Document 2: JP-A-2004-218183